1. Field of the Invention
The invention pertains to apparatus for positioning ships at sea in a predetermined relative manner and more particularly to a short pulse baseband radar and communication system for accurately positioning one ship in a predetermined relative position to that of another ship.
2. Description of the Prior Art
To protect our offshore regions and beaches, tanker ships from Alaska delivering oil to California ports, deposit their cargo in a storage ship anchored distal to the California coast. Attached to the stern of the storage ship is a flexible type pipeline with an oil receptacle at one end supported by a buoy at a distance from the storage ship. This flexible pipeline is used to funnel the oil from the shuttle ship into the tanks of the storage ship. Stored oil is then pumped from the storage ship's tanks to the mainland via a submerged pipeline. To expeditiously transfer the oil from the shuttle ship to the storage ship, it is necessary for the shuttle ship to accurately locate the storage ship and its buoy and position itself to accomplish this transfer with a minimum probability of oil leakage, especially under bad (foggy) weather conditions.
Prior art electronic docking systems such as that disclosed in U.S. Pat. No. 4,063,240 issued Dec. 13, 1977 to Isbister et al and assigned to the assignee of the present invention, are utilized for docking ships adjacent to jetties that are permanently located on shore. In these systems position information is generated by equipment located on the jetty and relayed to the ship for display along with docking commands from the jetty master. A beam signal is radiated from the docking ship and received by a monopulse system to determine the relative angular position of the beacon and by a Doppler system that measures the ship's velocity. A plurality of short baseband pulse radar systems are positioned along the jetty to determine the aspect of the docking ship relative to the jetty. Since the accuracy and repeatability of monopulse systems are sensitive to multipath signals, frequent boresighting and calibration of the equipment is required. This procedure being most frequently necessary on busy jetties where system downtime can least be tolerated.